In the MEMS nozzle arrangement described in U.S. Pat. No. 6,243,113 “Image Creation Method and Apparatus” (the contents of which are incorporated herein by cross reference), an ink chamber is provided with an ink inlet and an ink ejection port, which are coaxial. The ink ejection port is provided through thermal actuator that incorporates a paddle mounted to a substrate by a passive anchor and an active anchor. The active anchor includes a resistive element that heats up upon application of a current. This heating causes expansion of the active anchor, whilst the passive anchor is sufficiently shielded from the generated heat that it remains the same length. The change in relative lengths of the anchors is amplified by the geometric position of the anchors with respect to each other, such that the paddle can selectively be displaced with respect to the ink chamber by applying a suitable drive current to the active anchor.
Upon actuation, the paddle is urged towards the ink chamber, causing an increase in pressure in the ink in the chamber. This in turn causes ink to bulge out of the ink ejection port. When the drive current is removed, the active anchor quickly cools, which in turn causes the paddle to return to its quiescent position. The inertia of the moving ink bulge causes a thinning and breaking of the ink surface adjacent the ink ejection port, such that a droplet of ink continues moving away from the port as the paddle moves back to its quiescent position. As the quiescent position is reached, surface tension of a concave meniscus across the ink ejection port causes ink to be drawn in to refill the ink chamber via the ink inlet. Once the ink chamber is full, the process can be repeated.
One difficulty with the arrangement described in this nozzle arrangement (and similar systems) is balancing the compressive and bend loads on the active anchor. If the anchor is made too stiff relative to the force it can exert during self-heating, then it will not bend and the operative end of the paddle will not move sufficiently to eject ink. Alternatively, if the stiffness is too low, then energy will be wasted in bending the anchor. Even if the stiffness of the anchor is correct in a particular design, the energy lost to bending the active and passive anchors cannot be recovered. It would be desirable to provide an arrangement which does not rely on bend actuation as its primary provider of force to a paddle or other actuator.